The present invention relates to combustion systems, and more particularly, to a device and method for inhibiting formation of oxides of nitrogen (NO.sub.x) in a two-zone diffusion flame of a combustion system of a gas-fired furnace.
As a result of the combustion process, combustion systems normally generate gaseous combustion products which include oxides of nitrogen (NO.sub.x) which are vented to the atmosphere as flue gases. It is desirable to limit these NO.sub.x emissions since NO.sub.x is considered a pollutant and combustion systems for gas-fired furnaces sold in certain geographical areas must meet strict NO.sub.x emission standards.
It has been observed that the two most important parameters in the formation of NO.sub.x are peak combustion temperature and residence time at peak temperatures. One technique for limiting NO.sub.x emissions by controlling peak combustion flame temperatures is to provide a supplemental air flow for cooling the combustion flame in a combustion system. Another technique is to alter the combustion system to minimize the formation of NO.sub.x. However, these techniques, while limiting NO.sub.x formation, may adversely affect the combustion process of the combustion system by causing incomplete combustion and/or by adversely affecting the combustion process in other ways. Also, these techniques may require a major redesign of certain components of the combustion system, such as a redesign of burners and combustion chambers or heat exchangers of the combustion system, thereby rendering these techniques undesirable for retrofitting existing combustion systems. Another technique used with two-zone mono-port inshot burners, which generally emit a lower level of NO.sub.x emissions than multi-port burners, is to position a piece of material at the periphery of the secondary flame of the two-zone diffusion flame to temper the combustion flame by absorbing thermal energy from the combustion flame. In this technique, the device absorbs thermal energy from the combustion flame at a rate which inhibits formation of NO.sub.x while allowing substantial complete combustion of the fuel supplied to the burner. These devices are generally cylindrical screens juxtaposed to the periphery of the cooler, secondary combustion zone. Also, these devices aerodynamically smooth at least a portion of the periphery of the combustion flame, thereby inhibiting formation of eddies by near-stoichiometric mixtures of combustion substances at the periphery of the two-zone diffusion flame which are capable of forming relatively large amounts of NO.sub.x.